Elevated deoxycholate production is a positive risk factor for the development of colon cancer as well as cholesterol gallstones. However, the factors that regulate deoxycholate production are poorly understood. Only a few bacterial strains (specific members of the Eubacterium and Clostridium genera) carry the bile acid inducible (bai) operon that encodes the enzymes responsible for bile acid 7a-dehydroxylation, and these organisms constitute only a tiny fraction of the intestinal flora. While deoxycholate production correlates directly with intestinal flora content of bai operon-containing bacteria, little is know about what factors control the intestinal content of these organisms. We have recently developed a mouse model of primary bile acid malabsorption, the ileal bile acid transporter (Slc10a2) knockout mouse. Slcl0a2 null mice in a 129S6/SvEv background develop severe bile acid malabsorption. In contrast, crossing the Slcl0a2 null allele into a C57BL/6J background produced only bile acid malabsorption accompanied by increased deoxycholate production. This is the first example of the host genetic background apparently regulating the colonic content of deoxycholate-producing bacteria. The goal of this R21 application is to develop a mouse model system to study the genetic and environmental factors that regulate deoxycholate production. Three specific aims are proposed. Aim 1: To test the hypothesis that differences in the gut flora deoxycholate production are responsible for bile acid absorption differences between mouse strains lacking the ileal bile acid transporter (Sic10a2). The mouse strain gut flora differences will be assessed using antibiotics and bile acid analogs to measure gut flora differences in deoxycholate production. Aim 2: To develop enzyme and PCR-based assays to directly quantify the gut flora bacterium harboring the bai operon. Many of the genes responsible for the complex pathway that converts cholate to deoxycholate are located on a single bile acid inducible (bai) operon. In human colonic and fecal samples, cholic acid 7a-dehydroxylation activity has been directly measured and the bai genes detected by PCR. The goal of this aim is to apply these assays to the mouse to quantify host strain differences in bile acid 7a-dehydroxylation by the enteric flora. Aim 3: To cross the Slcl0a2 null allele into additional mouse strains as a first step toward cloning the genes responsible controlling deoxycholate production (colonization by the bai operon bacteria).